Solvent dependent mechanistic pathways for η-O2CCH3 substitution from the [Mo3(μ3-O)2(μ-O2CCH3)6(η-O2CCH3)3]− anion

Abstract

Rate coefficients for terminal acetate (η-O2CCH3) substitution from the [Mo3(μ3-O)2(μ-O2CCH3)6(η-O2CCH3)3]− anion were examined in polar protic (D2O and methanol-d4), polar aprotic (DMSO-d6 and DMF-d7) and acidic solvents (acetic acid-d4). Reaction rates were determined using variable-temperature 1H NMR and found to span orders of magnitude depending on the solvent (k298 K (s−1)=7.2×10−6 for D2O, 2.6×10−5 for methanol-d4, 3.7×10−4 for acetic acid-d4; no reaction for DMSO-d6 and DMF-d7). Unlike D2O and methanol-d4, little to no reaction occurs in DMSO-d6 and DMF-d7 most likely because the aprotic solvent is not able to solvate the leaving acetate ligand through hydrogen bonding. Activation parameters show that acetate substitution in polar protic solvents follows a dissociative pathway (D2O: ΔH‡=126 (±6) kJmol−1 and ΔS‡=80 (±18) Jmol−1K−1; methanol-d4: ΔH‡=115 (±3) kJmol−1 and ΔS‡=52 (±9) Jmol−1K−1) while parameters for acetate exchange in acetic acid-d4 are markedly different and suggest a mechanism more complex than a simple ligand exchange reaction (ΔH‡obs=63 (±12) kJmol−1 and ΔS‡obs=−97 (±40) Jmol−1K−1).